1. Field of the Invention.
The present invention relates to an apparatus for using a laser beam to record and/or read information, and more particularly, to an optical pick-up device applied to, for example, optical disk systems.
2. Description of the Related Art.
Optical memory system requirements are increasing, and relatively small optical pick-up devices of a low cost and high performance are in demand. The present invention presents an optical pick-up device for satisfying this demand. A conventional optical pick-up device for optical disk systems typically consists of a semiconductor laser, an optical lens for focusing the laser beam of the semiconductor laser onto an optical disk, a beam splitter for guiding a part of the laser beam reflected from the optical disk onto photodetectors, and a cylindrical lens for generating an astigma aberration in the laser beam reflected from the optical disk (Nakamura et al., National Tech. Report, Vol. 32, 490, 1986). Since a conventional optical pick-up device requires several optical components, as mentioned above, the size of the optical pick-up device is relatively large and the production costs, which include the cost of complicated optical axis adjustment, is high. In order to reduce the number of such optical components and to realize a relatively small and low cost optical pick-up device, a holographic type optical pick-up device, which employs a hologram which functions as both a beam splitter and cylindrical lens, was proposed by Kimura et al. (Proc. of the 22th Micro-Optics Conf. Vol. 14, 228, 1986). The schematic configuration of a proposed holographic type optical pick-up device is illustrated in FIG. 6. A laser beam emitted from a semiconductor laser 61 passes through a hologram 62, and is focused onto an optical disk 64 by a focusing convex lens 63. The laser beam is reflected off of the disk surface, the intensity of the reflected laser beam being in accordance with the recorded information, and the reflected laser beam returns towards the semiconductor laser 61 through the focussing lens 63 and the hologram 62. In this process, a part of the reflected laser beam is diffracted into two directions by the hologram 62 having the two regions R1 and R2, and the diffracted beams are respectively guided and focused onto a quadrant photodetector 65. By signal processing of the photo current of the quadrant photodetector 65, the recorded signal and focusing error and tracking error signals are detected.
In this conventional optical pick-up device, however, when the laser beam which is emitted from the semiconductor laser 61 passes through the hologram 62, a part of the laser beam power (typically 50%) is diffracted and lost. The transmitting power, which is utilized for reading and recording operations, and the power of the beam focused onto the photodetector, which affects the SN ratio during signal detection, are dependent on the diffraction efficiency. When the diffraction efficiency of the hologram 62 is high, the transmission efficiency is low, and the laser beam power focused onto the disk surface is thus relatively small, rendering it difficult to realize a sufficient power level to write or read information. On the other hand, when the diffraction efficiency is low, the laser beam power onto the photodetector is relatively small, rendering it difficult to obtain signals of a sufficiently high SN ratio. A diffraction efficiency of 50% is optimal for realizing the highest SN ratio. Moreover, in the latter case of the low diffraction efficiency, since the feedback power to the semiconductor laser is relatively large due to the low diffraction efficiency, the semiconductor laser oscillation may possibly become unstable and cause an increased intensity in fluctuation noise.
This invention is presented to overcome these problems and to provide a highly efficient, low noise, low cost, and small size optical pick-up device.